1. Field of the Invention
The present invention relates generally to X-ray windows with coated silicon or silicon-compound support structures.
2. Related Art
Radiation windows are used in radiation-detection systems, which in turn are used in detecting and sensing emitted radiation in connection with electron microscopy, X-ray telescopy, and X-ray spectroscopy. Radiation-detection systems typically include in their structure a radiation window, which can pass radiation emitted from the radiation source to a radiation detector or sensor. The radiation detection system's design constraints define the radiation window's performance requirements.
A radiation window's performance is measured in part by its ability to transmit lower-energy X-rays while still being able to withstand a pressure differential with minimal or undetectable vacuum leak rates. Standard radiation windows typically comprise a sheet of material placed over an opening or entrance to the detector. As a general rule, the sheet-material's thickness, density, and mass absorption coefficient correspond directly to its ability to pass radiation. The most desirable materials for maximizing radiation transmission include the least dense and lowest mass-absorption coefficient materials which are typically the lowest atomic-mass elements. It is also desirable to provide a sheet of material that is as thin as possible, yet capable of withstanding cracks, pinhole voids or other breaks leading to leakage resulting from gravity, vibration, normal wear and tear, and differential pressure. It is also desirable in many applications to block visible light. Veli-Pekka et al. in “Comparison of Ultrathin X-Ray Window Designs” compare the radiation transmission performance and leak-rate performance of several materials and sheet-material configurations. The authors note that “windows which do not have to withstand atmospheric pressures can be supported by a polyimide grid” but further clarify that “by using a separate tungsten support grid as an additional support, the polymer grid supported windows also can be made pressure tolerant.” See Viitatanen, Veli-Pekka et al., “Comparison of Ultrathin X-Ray Window Designs,” Journal of X-Ray Science and Technology 4, (1994): 182-190.
As Veli-Pekka et al. note the need to minimize thickness in the membranes used to pass radiation they suggest a solution to support the thin sheet of material with a tungsten grid support structure. Known support structures include frames, screens, meshes, ribs, and grids. While useful for providing support to an often thin and fragile sheet of material, many support structures can interfere with the passage of radiation through the window assembly due to the structure's composition, geometry, thickness, and/or height.
Higher atomic-mass elements used in the radiation window's sheet material and support structure composition often cause spectral contamination when used in X-ray spectrometry or fluorescence-type applications. In X-ray fluorescence spectrometry, photonic radiation from a primary radiation source irradiates the atoms of a specimen, expelling electrons from the atoms' inner-orbital shells creating shell vacancies. Outer-orbital electrons “fall” into the inner-orbital shell vacancies emitting secondary radiation equal to the difference in energy between the outer and inner shells. This discrete emitted radiation, or spectral emission, is unique to the fluoresced element and is measured in electron volts (eV). A radiation detector converts the discrete spectral emissions into a voltage signal which is then fed into a pulse processor and multi-channel analyzer. The pulse processor and multi-channel analyzer produce a histogram to rank and quantify the distinct energy values emitted from the specimen. As each photon (secondary radiation) is received into the radiation-detector sensor, the photon's energy (eV) is measured and a count is placed in an energy “bin” corresponding to its value. Photons with identical energy values are counted into the bin. The more counts a specific bin receives, the more prevalent the corresponding element is in the specimen. The result is a series of Gaussian (bell-curve) peaks, displayed on a software-generated graph, corresponding to the elemental composition of the specimen. An energy-dispersive detector is said to have high resolution when it is able to readily distinguish between the spectral counts (or photonic energy eV counts) to produce well-defined Gaussian peaks representing the elements comprising the specimen. Detector resolution is defined as the fall-width eV span at half the maximum height of a known Gaussian curve (FWHM).
Good spectral resolution is more difficult to achieve when measuring a specimen comprised of lower atomic-number elements. Secondary radiation from low atomic-number elements, referred to as soft X-rays and measured by the energy-dispersive detector, is low in energy and thus more easily absorbed by the detector's radiation window and window support structure. If absorbed, the energy is not received by the radiation-detector sensor and thus not counted by the pulse processor and multi-channel analyzer, creating an erroneous composition calculation.
Spectral energy or radiation from a fluoresced specimen may in turn fluoresce the radiation window's components creating a tertiary radiation. The tertiary radiation enters the sensor as the characteristic radiation of the window's components instead of the specimen's characteristic radiation. The result is the confounding of the specimen's spectral energy into unknown or other elements' spectral-emission lines, called ‘stray lines.’ This tertiary fluorescence mixes with and contaminates the specimen's emission signal causing the detector to erroneously calculate the sample's elemental composition. This is especially true if the radiation window's sheet material and support structure composition comprise higher-atomic number elements or if the detector is used “to detect minute concentrations of trace elements.” (See F Scholze and M. Procop, “Detection efficiency of energy-dispersive detectors with low-energy windows,” X-Ray Spectrum 34, (2005): 473-76.) For these reasons, using the lowest atomic-mass elements in the radiation window's sheet material and support structure composition provides the best radiation window suitable for high-performance energy-dispersive radiation detectors.
The window material is typically very thin and flexible and requires a support structure to span the window-opening area. The support structure thickness also plays an important role in optimizing radiation transmission. For example, in energy-dispersive radiation-detector applications, the fluoresced specimen under examination emits radiation in all directions. Typically, the specimen, the radiation detector window, and the detector sensor are located as close as possible to each other to decrease radiation travel distance and increase the amount of radiation entering the detector sensor. Only photons traveling in a direct line-of-sight between the fluoresced element and the radiation detection sensor will enter the sensor without hitting the support structure's ribs or grid sides. If the radiation window's support structure is excessively tall, a significant portion of radiation will be absorbed into the ribs' side walls, effectively collimating radiation that would otherwise be measured. Decreasing the support structure's height increases the line-of-sight opening between the radiation source and the sensor. In X-ray spectrometry applications, reduced-height support structures minimize radiation collimation, increase radiation flux, and thus decrease sensor reading times.
Silicon can be an ideal material for use as a radiation window's support structure because silicon is a low atomic-mass element that minimizes spectral contamination. Silicon is also ideal because it is easy to etch suitable window frames with well-defined edges and flat sidewalls using established etching techniques. However, silicon is a brittle and relatively weak material for use as a structural support member. The radiation window support structure must be sufficiently robust to withstand pressure differentials and vibration which occur under normal use. The radiation window is a comparatively inexpensive part of the radiation detector assembly but the radiation window's failure may result in catastrophic damage and require total replacement of the radiation detector.
It has been proposed to strengthen silicon needles by growing and later removing a silicon-oxide layer in order to provide a smoother surface with fewer defects, where the surface defects would otherwise initiate microcracks and thus decrease fracture toughness. See Smart et al in U.S. Pat. No. 6,852,365.
It has been proposed to dope a silicon substrate with boron as part of an etching process. See U.S. Pat. No. 4,960,486.
The radiation-window support-structure geometry contributes significantly to its ability to transmit radiation. Support-structure geometry defines the number of ribs or grid density as well as the width of the individual ribs or grid walls. Higher-frequency rib count, higher-grid density, or wide grid-wall structures decrease the amount of unobstructed open area, which decreases radiation transmission. The need for increased structure-free window area should be balanced with the need to support stretchy polymer-based window film. Support ribs spaced too far apart or grid density that is too low may cause unacceptable window-film deflection when the window must withstand a pressure differential.